Amplifying system



Apvfi 21, 1936. l. WOLFF 2,037,842

AMPLIFYING SYSTEM Original Filed April 25, 1931 Fig.1.

INVENTOR. Irving Wolf-F,

FwmNA .5/ A r HIS ATTORNEY.

Patented Apr. 21, 1936 V 2,037,842 AMPLIFYING SYSTEM Irving Wolff, Merchantville, N. J., assignor, by mesne assignments, to Radio Corporation of America,'New York, N. Y., a corporation of Delaware Application April 25, 1931, Serial No. 532,778 Renewed May 23, 1933 14 Claims. (01. 179-1) My invention relates to amplifying systems. istics of the sound output therefrom that a and, more particularly, to systems designed for change in volume is unaccompanied by an apthe amplification of currents at radio and audio parent change in fidelity of reproduction. frequencies. There is another psychological phenomenon It has been observed that although a loudwhich relates to the present invention and which speaker associated with radio receiving appaenables us to hear low notes which are entirely ratus or the like may sound quite natural, when missing from the sound reproduction. This phethe acoustic intensity is medium, speech becomes nomenon results from the fact that the human tubby when the intensity is greatly increased ear is unable to detect any change in pitch of and thin when the intensity is decidedly rea note if the fundamental and lower harmonics 10 duced. Even when the most perfect amplifiers, are removed. In fact, the pitch is recognizable as heretofore constructed, are operated well beif but three consecutive higher harmonics relow the overload point, there is an apparent main and, if proper control of the harmonic frechange in the fidelity of sound reproduction quencies of the lower notes is had, the natural- 15 with change in sound-output intensity, the pheness of the loudspeaker response may be pre- 5 nomenon not being a result of the characterisserved through volume changes, even though the tics of the amplifier but being occasioned by the fundamentals are missing. frequency-pressure-sensitivity characteristics of It is, therefore, an ancillary object of my inthe human ear. vention to provide an amplifying system that In other words, the ear is relatively less senshall be of value in improving the apparent low 20 sitive to low frequencies at low intensity than frequency response, at low intensities, of loudit is when the volume is raised and, at low inspeakers. tensity, a much smaller increase in sound-pres- According to my invention, one of the thersure is required for low than for high frequenmionic devices included in an audio frequency cies to give the same increase in apparent loudamplifier is provided with an input circuit con- 25 ness. Furthermore, at low intensity, if the high stituted by a resistance-reactance network and and low frequencies are to give equal apparent with means whereby audio frequency potentials loudness the sound-pressure occasioned by the may be impressed, at substantially constant amlow frequencies must be many times that from plitude, across the input terminals thereof from the high frequencies. At very high intensities, a preceding stage such as the detector in a radio 30 however, the same sound-pressure causes subreceiver. I also provide means whereby one of stantially the same loudness at all frequencies the resistors in the network may be utilized as within the acoustic range. a potential dividing device to control the am- Having the above facts in mind, it is clear plitude of the potentials impressed upon the amwhat happens when the usual volume-control of plifying tube and so arrange and dispose the-seva radio receiver, a phonograph amplifier or the eral reactors that the rate of control of the polike is advanced past the normal position. Sound tentials at frequencies in one range exceeds the pressures set up by the loudspeaker at all frerate of control of the said potentials at frequenquencies are increased in the same proportion cies in a. different range. In sh0rt,'the network but the same proportionate increase in sound referred to is such that when the amplitude of 40 pressure means a much greater apparent inthe potentials impressed upon the amplifying.

crease in loudness at the lower frequencies and, tube is increased, corresponding to an advance therefore, the reproduction assumes the tubby of the volume-control, the potentials at low fresound mentioned above and the converse is true quencies are preferentially attenuated in order when the intensity is below normal. that the ultimate sound output shall not be 45 It is, accordingly, an object of my invention ftubby. to provide an amplifying system that, when it I also may include reactance elements of other is utilized to energize a sound reproducer, shall types such as tuned circuits in the network in give rise to natural sound impressions on the order that the amplifier shall have a peaked reear irrespective of changes in the volume-level sponse atfrequencies corresponding to the har- 30 introduced by a volume-control device. monies to which I referred previously. Another object of my invention is to provide In addition, when applying my invention to a volume-control for an amplifying system that radio receiving apparatus, I find it preferable to shall function to maintain such correspondence provide means for raising the average audio frebetween volume-level and frequency characterquency output from the detector tube, automatically, to a predetermined standard value before setting the acoustically compensated volume-control device into operation. This provides a definite minimum low intensity sound output from the apparatus that is independent of the intensity of the received signal. Further increase in sound-intensity is secured by means of the volume-control device which is so compensated as to keep the apparent fidelity of reproduction constant at all sound intensities.

A still further and more specific object of my invention, therefore, is to provide a control-device particularly adapted for use in amplifying systems of the aforementioned type.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in connection with the accompanying drawing, in which Fig. 1 is a diagrammatic view of a portion of a radio receiving system constructed according to my invention,

Fig. 2 is a perspective view of a control-device of preferred type,

Figs. 3 and 4 are diagrammatic end views of the control-device, indicating the successive positions it assumes in operation, and

Fig. 5 is a diagrammatic View of a circuit modified according to one phase of my invention to provide peaked amplifier response at low audio frequencies.

Referring to Fig. 1 of the drawing, an amplifying system constructed according to my invention preferably includes one or more stages (not shown) for amplifying an incoming signal at radio frequency, a detector stage I and at least one audio frequency amplifying stage 3.

The radio frequency amplifier is provided with automatic or manually actuated volume-control devices exemplified, in the drawing, by a variable resistor 5 connected in shunt relation to an inductor I included in the output circuit of the amplifier. Further reference to the variable resistor will be made in describing the operation of my improved system.

A thermionic tube 9, included in the detector stage, is provided with an input inductor ll,

coupled to the output inductor 1 of the radioquency transformer.

frequency amplifier and with an output circuit including the primary winding I3 of an audio fre- Detection may be secured by interposing a grid condenser 15, shunted by a grid leak H, in the input circuit, as shown, or the tube may be biased to substantially the cutoff point, to give plate-rectification, instead.

The terminals of the secondary winding IQ of the audio-frequency transformer are connected to a resistance-reactance network from which variable potentials may be impressed between the grid and filament of a thermionic tube 21 which functions as an audio-frequency amplifier. Additional stages of audio frequency amplification, of any desired type, may be added if necessary.

As previously pointed out, the network interposed between the detector and the first audio frequency amplifier, according to my invention, has two primary functions, namely, control of volume of sound-output and compensation of the said output in accordance with the frequencypressure sensitivity characteristics of the ear. The desired result is substantially attained if the potentiometer of usual type.

network includes means for simultaneously increasing, through the actuation of a volume-control device, the average audio potentials applied to the amplifier tube 2| and causing the potentials at high audio frequencies to increase at a rate exceeding the rate at which potentials at low audio frequency are increased.

The volume-control device per se is constituted by a potentiometer, the movable contact arm 23 of which is connected to the grid of the amplifying tube 2| and the terminals of the series connected resistance elements R1 and R2 of which are, respectively, connected to the cathode of the tube and a terminal of the secondary winding l9 through one or more fixed resistors R R5, and to the opposite terminal of the secondary winding through a condenser C1. An intermediate point between the resistance elements is-connected also to the last mentioned secondary winding terminal through a fixed resistor R2. The fixed resistor R5 between the resistance element R3 and the cathode of the tube 2|, is provided with a shunting condenser C2.

For best results the volume-control impedance, at any setting, should be smaller than the input impedance of the amplifier tube whereby it functions substantially the same asan unmodified The impedance of the resistor R5 should be large as compared to that of the condenser C2.

In general, the relations among the several circuit elements, in a preferred commercial embodiment of my invention may be expressed by the following formulae:

5. Impedance of C 360 cycles=impedance of From a consideration of the foregoing formulae, and of the drawing, it will be clear that a movement of the potentiometer arm away from the mid-position thereof, corresponding to normally loud sound, for volume-control, is accompanied by a change in the ratio of high to low frequency potentials appearing between the grid and cathode of the amplifier tube.

That is to say when the ultimate current output volume is increased by moving the arm 23 toward the upper end of the resistance element R1, the effect of the condenser C1, is increased with the result that the potentials at high frequencies impressed on the amplifying tube 2| become relatively greater than potentials at low frequencies, thus counteracting the tendency for the sound-output to become tubby as hereinbefore mentioned. This action may be seen by considering the potential drop between the contact arm and the point of connection of R2 to R1. Since R2 has approximately of the resistance of 'R1, the potential of the point of connection is not affected materially by the increased impedance of C1 at the lower frequencies. If, however. the

contact arm is near the juncture of R1 and C1 it will be evident that at high frequencies C1 has low impedance, thus allowing the full potential available from the secondary of the transformer to be impressed on the grid of the tube, while at low frequencies, C1 has a material impedance compared to the resistance of R1 and that, therefore, the potential on the grid of the tube is decreased due to the drop in potential over the condenser C1.

Conversely, when the contact arm is moved toward the lower end of the resistance element R3 the condenser C1 becomes of less effectiveness as-a coupling element for high frequencies while the condenser C2, the reactance of which, at high frequencies, is low, by-passes the high frequencies to a certain extent. At lower than normal sound intensities, therefore, the high frequencies are attenuated in favor of the low frequencies and the objectionable thinness of the reproduced sound is eliminated.

In order that the sound output from the system shall be definitely related to the setting of the volume-control device irrespective of the differences in carrier amplitude which exist among transmitting stations, I have found it desirable to provide means whereby the average potentials appearing across the secondary winding of the audio frequency transformer may be adjusted to a constant pre-determined volume.

For this purpose I prefer to provide either automatic or manually adjustable volume-control means in the radio frequency amplifier and automatic limiting means associated with either the input or the output circuit of the detector tube.

In the modification of my invention chosen for purposes of illustration, the volume-control means in the radio frequency amplifier is constituted by the variable resistor 5 connected in shunt relation to the output circuit of the said amplifier. The limiting means associated with the detector comprises a thermionic tube 25 having an input circuit constituted by a resistor 21 and an output circuit including the winding of a relay 29. The armature of the relay is constructed in the form of a detent device 3| for a purpose which, hereinafter, will be explained in more detail. A biasing battery 33 is included in the input circuit of the limiter tube, the potentials thereof being such as to bias the tube to substantially the cut-off point.

The input resistor 21 is connected in shunt relation to the primary winding l3 of the audio frequency transformer, in series with a by-pass condenser 35.

Referring now to Fig. 2 of the drawing, the volume-control resistor 5 in the radio frequency amplifier and the resistance elements R1, R3, included in the network between the detector tube and the amplifier tube, are disposed concentrically with respect to an actuating shaft 36. The resistor 5 is loosely journaled upon the shaft 36 while the resistance elements R1 and R3 are rigidly mounted on a supporting base 31.

The shaft carries a contact arm 38 which engages the resistor 5, as shown in Fig. 1, and also the contact arm 23. The contact arm 38 is rigidly attached to the actuating shaft, while the contact arm 23 is rigidly attached to the resistance element 5 through any convenient means such as a U-shape bridging member 39 and is frictionally engaged with the shaft. The frictional engagement of the arm 23 with the shaft may be providedby a split collar 4| having an adjusting screw 43. The detent device 3| is so disposed with respect to the bridging member as to maintain the contact arm 23 at the low potential end of the resistance element Rs in the absence of energizing current in the relay winding. a

In the operation of the volume-control device, the shaft, through manipulation of a knob 45 afllxed thereto, is first turned completely to the left, as shown in Figs. 1 and 2. The detector tube is, accordingly, deprived of input potentials and no radio frequency potentials appear across the input resistor 21 of the limiter tube 25. In this position the bridging member 39 engages and is held by the detent device 3|.

Upon slowly turning the knob to the right, as indicated by the arrow, the arm 38, engaging the resistor 5 in the radio frequency amplifier, moves with respect to the said resistor, thus causing an increase in the input potential applied to the detector tube 9. Since the output, at radio frequency, from the detector tube, is impressed across the input resistor 21 of the limiter tube 25, the plate current of the latter tube, flowing in the winding of the relay 29, is representative of the amplitude of the audio signal delivered to the secondary winding id of the audio frequency transformer.

By proper adjustment of the various circuit constants, therefore, the relay 29 may be caused to retract the detent 3| at the instant when the output from the detector is just sufficient to give minimum response from the loudspeaker quot shown). One possible setting of the control device at the moment the relay operates is indicated in Fig. 3 of the drawing.

As soon as the detent device is retracted, the contact arm 23 is free to turn with respect to the resistance elements R3, R1 to increase the audio output to the desired point. Inasmuch, however, as rotary motion is transmitted to the resistor 5 in the radio frequency amplifier through the bridging member 39, the said resistor and the contact arm associated therewith move as a unit during the continued adjustment of thevolumecontrol device, as indicated in Fig. 4, in order that the input to the detector tube shall remain substantially constant during the reception of signals from any given station.

In the event that it is desired to receive signals from some other transmitting station, the control device is reset to zero position and the sequence of operation just described is again followed.

Under certain circumstances it may be desirable to so modify the amplifier that it shall have a peaked response at frequencies in the neighborhood of 200 cycles per second in order that the apparent response of the sound producing device shall be greater at frequencies below 200 cycles. This result may be accomplished, as shown in Fig. 5 of the drawing, by connecting the resistance element of a volume-control potentiometer 41 in series with a resistor 49 and a. circuit 5| tuned to the frequency at which peaked response is desired. At high volume-settings of the potentiometer, the effect of the tuned circuit is negligible, while at low volume-settings its reactance becomes appreciable.

It will be apparent from the foregoing description of my improved amplifying system that, by its use I am enabled to greatly improve the realism of sound reproduced through a radio receiving set. My system is also advantageous in that the sound output therefrom is always proportional to the setting of the volume-control de- Lil vice irrespective of the strength of the incoming radio signals.

Although I have illustrated and described certain proferred embodiments of my invention, numerous modifications thereof will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In an amplifying system, a thermionic tube having an input circuit including a potential dividing device, means for impressing audio frequency potentials upon said device, means for utilizing said device for controlling the potentials impressed across the input terminals of the thermionic tube, means directly connected in circuit electrically with said device for simultaneously attenuating potentials at frequencies lying in a predetermined range, and means responsive to signal potentials above a predetermined amplitude for controlling an operation of said device.

2. In an amplifying system, a thermionic tube having an input circuit including a potential dividing device, means for impressing audio frequency potentials upon said device, means for utilizing said device for controlling the potentials impressed across the input terminals of the thermionic tube, means directly connected in circuit electrically with said device for simultaneously attenuating potentials at frequencies lying in a predetermined range, and means for maintaining substantially constant the average audio frequency potentials impressed upon the potential dividing device.

3. In an amplifying system, a radio frequency amplifier, an audio frequency amplifier, volumecontrol means associated with said radio frequency amplifier, a second volume control means associated with said audio frequency amplifier, common means for operating said volume control means independently of each other, high audio frequency attenuating means connected with said audio frequency volume control means whereby it operates simultaneously as a tone control means and volume control means, and means operative in response to signal amplitudes above a predetermined value for selectively controlling the operation of said volume control means.

4. In a signal amplifying system, a thermionic tube having an input circuit including a potential dividing device, means for impressing audio frequency potentials upon said device, means for utilizing said device for controlling the potentials impressed across the input terminals of the thermionic tube, means for simultaneously attenuating potentials at frequencies lying in a predetermined range, and means for sequentially controlling (l) the amplitude of the audio frequency potentials impressed upon the potential dividing device and (2) thepotentials impressed across the input terminals of the thermionic tube.

5. In an electrical amplifying system including a detector, the combination of volume control means connected in circuit preceding and succeeding said detector, said means being associated with circuits other than the detector circuit, means associated with the volume control means in circuit, succeeding the detector for attenuating the high frequency end of the audio frequency range at low volume and for attenuating the low frequency end of the audio frequency range at high volume, means for operating said volume control means in sequential relation to each other, and means responsive to signals of an amplitude exceeding a predetermined value for controlling the transition from the one volume control means to the other in said sequential operation.

6. A volume control means for a signal amplifying system including, in combination, two variable potentiometer type resistors each hav ing a resistor element and a contact relatively movable therewith, means providing a fixed support for one of the elements of one of said variable resistors, the other element of said variable resistor and an element of the second variable resistor being mechanically connected and movable together, means for'releasably holding said mechanically connected elements in a fixed position, means responsive to signals of an amplitude exceeding a predetermined value for operating said releasable holding means, whereby said elements are movable, means for moving the remaining element of said second variable resistor, and means providing a frictional connection between said last named element and said mechanically connected elements.

'7. In a signal transmission circuit including a plurality of cascade connected thermionic devices, the combination of a volume control device in circuit preceding one of said thermionic devices and a second volume control device in circuit succeeding said thermionic device, a tone control compensating circuit connected with said second named volume control device, and common means for sequentially operating said volume control devices.

8. In a signal amplifying system, a vacuum tube amplifier device having an input circuit including a tapped potential dividing device, automatic volume control means for impressing audio frequency potentials upon said device at predetermined amplitudes, means connected with a separate tap point and one terminal of said potential dividing device for attenuating potentials at audio frequencies lying in a predetermined audio frequency range, and a tap connection movable along said potential dividing device Within and without the section shunted by said last named means for controlling audio frequency potentials impressed across the input terminals of the thermionic tube simultaneously in amplitude and frequency characteristic.

9. In a signal amplifying system, a vacuum tube amplifier device having an input circuit including a potential dividing device, automatic volume control means for impressing audio frequency potentials upon said device at predetermined amplitudes, means connected in shunt with a fixed portion of said potential dividing device, and means connected with a variable portion of said potential dividing device for simultaneously attenuating potentials at audio frequencies lying in a predetermined audio frequency range and controlling audio frequency potentials impressed across the input terminals of the thermionic tube.

10. In an audio frequency volume control system, a tapped volume control potentiometer device comprising a resistor element having at least two resistor sections, means providing a fixed tap connection between said sections, an electrical circuit impedance means connected in shunt with one of said sections through said tap connection, circuit means connected with said resistor element for supplying thereto signal potentials of substantially constant average amplitude, and an output circuit variably connected With said resistor element.

11. In an audio frequency volume control system, a tapped volume control potentiometer device comprising a resistor element having at least two resistor sections, means providing a fixed tap connection between said sections, an electrical circuit impedance means connected in shunt with one of said sections through said tap connection, means connected with said resistor element for applying signals of substantially constant average amplitude thereto, and an output circuit variably connected with said resistor element. I

12. In a radio receiving system, automatic volume control means for providing substantially constant average signal amplitude, an audio frequency signal transmission circuit for receiving signals controlled by said automatic volume control means, an audio frequency volume control means including a tapped resistor in shunt with said circuit, an output circuit having a variable tap connection with the resistor substantially throughout its length for volume control, an impedance network connected in shunt with a portion of said resistor including one end thereof, said network and said portion of said resistor having a predetermined impedance ratio where by tone compensation is provided simultaneously with volume control, the compensation increasing as the vollune control tap connection is moved to a position to reduce the volume.

13. In a radio receiving system having an audio frequency signal transmission circuit, the combination of means for supplying to said circuit audio frequency signals of substantially constant average amplitude, a variable audio frequency volume control means having an impedance element connected in shunt with said circuit, means providing a shunt impedance network in connection with a fixed portion of the impedance element of said volume control means, and an output circuit variably connected with said first I named circuit through said volume control means.

14. In a radio receiving system including an audio frequency signal transmission circuit, the combination of means for supplying to said circuit audio frequency signals oif substantially constant average amplitude, a variable audio frequency volume control means having an impedance element connected in shunt with said circuit, means providing a signal attenuating shunt impedance network in connection with a fixed portion or the impedance element of said volume control means including a terminal end, and an output circuit variably connected with said first named circuit through said volume control means, said last named variable connection being movable along said fixed portion of the impedance IRVING WOLFE. 

